The type of night vision system with which the present invention is concerned makes use of an image intensifier to convert small amounts of available light (for example starlight) by electron multiplication to an image which can be viewed by the human eye, recorded on film or video tape, etc.
A state-of-the-art night vision system of this character is disclosed in the '595 patent, and other systems more-or-less related to the system disclosed in the '595 patent are disclosed in various ones of the cited references and in the patents cited in column 1 of the patent.
A common disadvantage of heretofore night vision systems is that they are not capable of producing a visual image with sufficient detail under very low level ambient light conditions and/or when the target acquired by the system is very far away or the system has a slow objective lens.
A solution to this problem, disclosed in the '595 patent, employs an invisible light projector or spot illuminator to enhance the ability of the hand held night vision system disclosed in that patent to produce an acceptable visible image under such conditions. This light projector includes a diode which emits light in the invisible, near infrared part of the electromagnetic spectrum. This "invisible" light is focused and projected onto the target acquired by the night vision system. This provides, on that part of the target impacted by the projected beam, additional energy which can be converted by the image intensifier of the system to a visual image.
Although a decided improvement on the theretofore available night vision systems, that system disclosed in the '595 patent does have its drawbacks. The infrared LED is not capable of illuminating a target more than 500 feet (152 m) from the system to a high level; i.e., to a high enough level that the image generated by the intensifier tube will have any significant level of detail unless expensive optics with large apertures and low f numbers are employed.
Another limitation of the system disclosed in the '595 patent, which may be disadvantageous in particular circumstances, is that no provision is made for adjusting the "spot" or circle illuminated by the infrared LED with respect to the field of view of the night vision system. Therefore, the user may find that part of the target is adequately illuminated while the rest of it cannot be seen well at all. Alternatively, he might have to train the spot on first one and then another part of the target to see it well; and this is a decided drawback.